The present invention disclosed herein relates to a lithium secondary battery having enhanced energy density, and more particularly to a lithium secondary battery of which energy density and capacity are improved by reducing a final discharge voltage using various lithium mixed transition metal oxides as a cathode active material, or using the lithium mixed transition metal oxide with a lithium cobalt oxide (LCO)-based cathode material mixed.
In line with technology development and increasing demand for mobile devices, demands for a secondary battery as an energy source are sharply increased. Of various secondary batteries, many studies have been conducted on lithium secondary batteries with high energy density and discharge voltage, and lithium secondary batteries are commercialized and widely used.
Typically, unlike non-rechargeable primary batteries, secondary batteries which can be charged and discharged are actively studied with the development of state-of-the-art technologies such as digital cameras, cellular phones, notebook computers, and hybrid cars. Examples of secondary batteries may include nickel-cadmium batteries, nickel-metal hydride batteries, nickel-hydrogen batteries, lithium secondary batteries, and the like.
In general, performances required for secondary batteries may include high energy density, high output density, low self-discharge rate, high energy efficiency, and long cycle life. Therefore, among the plurality of secondary batteries, lithium secondary batteries have been known as a high-energy density battery because lithium secondary batteries have several advantages such as wide range of operating temperature, stable discharge voltage, and low self-discharge rate.
Also, since lithium secondary batteries have an operating voltage of 3.6 V or higher, they are used as a power supply for mobile electronic devices, or used for high power hybrid cars in a state that several lithium secondary batteries are connected in series. Lithium secondary batteries have an operating voltage three times higher than those of nickel-cadmium batteries or nickel-metal hydride batteries, and excellent energy density per unit weight, and therefore tend to be popularly used. Furthermore, with the development of mobile communication and information electronics industries, demands for lightweight lithium secondary batteries with high capacity continue to increase.
Therefore, researches on anode and cathode materials have been conducted to develop secondary batteries having the above-described performances. Of these anode and cathode materials, a cathode active material which is expressed as LiCoO2 has been representatively studied.
Most of currently used cathode active materials are a lithium cobalt oxide (hereinafter, referred to as ‘LCO-based cathode material’), and FIG. 1 is a graph showing a discharging profile of an LCO-based cathode material. It can be observed that a slope is very sharp at the end of discharge of the cathode material, which shows that a slight difference in capacity leads to a great difference in voltage.
It can be understood that a final discharge voltage of the LCO-based cathode material is 3.0 V; however, even if the final discharge voltage gets lower than 3.0 V, there is no change in capacity and energy density in the LCO-based cathode material. Moreover, the LCO-based cathode material has a limitation of small discharge capacity.
Technologies of partially replacing Co with another transition metal in LiCoO2 have also been studied. However, such an active material also has low energy density and poor cyclic properties, and thus those technologies are insufficient to obtain lithium secondary batteries having high energy density required in a market of secondary batteries. Therefore, it is required to develop a cathode material enabling capacity and energy density to be improved.